Among the conventional schemes of wireless communication equipment (mobile communication equipment) such as portable telephone units, one is GSM (Global System for Mobile Communication) which is adopted in European countries. The GSM scheme performs the phase modulation called GMSK (Gaussian Minimum Shift Keying) which shifts the phase of carrier wave in accordance with transmission data.
Generally, the transmission output stage of the wireless communication equipment incorporates a high frequency power amplifier circuit. Some GSM-based wireless communication equipment is designed to establish the communication output power level in need by controlling the bias voltage of the high frequency power amplifier circuit in accordance with the control voltage which is produced by the APC (Automatic Power Control) circuit based on the demanded transmission level from the baseband LSI and the signal from the transmission output detector.
In the field of recent portable telephone units, there is a proposal of the EDGE. (Enhanced Data Rate for GMS Evolution) scheme having a dual mode communication function, in which audio signal communication is performed based on GMSK modulation and data communication is performed based on 8-PSK (Phase Shift Keying) modulation.
The 8-PSK modulation is the phase shift of carrier wave derived from the GMSK modulation, with amplitude shift being added. It is capable of sending 3-bit information per symbol, in contrast to the GMSK modulation which sends 1-bit information per symbol. Accordingly, the EDGE scheme can perform communication at the higher transmission rate than the GSM scheme.
The high frequency power amplifier circuit of the GSM-based communication system can operate in the saturation region for amplifying the phase-modulated signal in accordance with the demanded output level, whereas the high frequency power amplifier circuit of the wireless communication system, which performs the EDGE-based transmission/reception, necessitates the amplitude control and therefore must have a linear operation in the non-saturation region.
For the high frequency power amplifier circuit of the communication system which is operative based on both the GSM scheme and EDGE scheme, a conceivable operational manner of the high frequency power amplifier circuit is to control the gate bias voltage of the output FET in accordance with the demanded output level, with the input signal amplitude being fixed, in the GSM mode which has GMSK modulation, and to control the output power by varying the input signal amplitude, with the gate bias voltage of the output FET being fixed, in the EDGE mode which has 8-PSK modulation.
However, this manner necessitates a variable-gain amplifier and its control circuit for varying the input signal amplitude in the EDGE mode, resulting in an increased circuit scale (refer to FIG. 2). Moreover, for the enhancement of modulation accuracy and power efficiency at a small output level, it is necessary to control the bias voltages of the variable-gain amplifier and the high frequency power amplifier circuit simultaneously, resulting in an intricate control system.
In the case of the fixed gate bias voltage scheme, with satisfactory linear characteristics being intended in the EDGE mode in which the high frequency power amplifier circuit must have a linear operation, a large idle current is needed to flow by setting such an output FET bias voltage as to have a higher gain relatively to the GSM mode. However, the gain is too high for a small input signal level, and a resulting amplified noise component gives rise to an increased noise leakage to the reception frequency band which is separated by 20 MHz or more from the transmission frequency.
In the wireless communication system, the high frequency power amplifier circuit has its output power controlled in accordance with the demanded output level from the control circuit (baseband circuit, etc.) and based on the feedback of the output of high frequency power amplifier circuit or antenna detected with a coupler and detection circuit. In this respect, the inventors of the present invention studied a current sensing scheme for detecting the output level from the output current of the high frequency power amplifier circuit within the semiconductor chip with the intention of making the circuit scale smaller. The study revealed a problem of this scheme when applied to the control of output power by varying the input signal amplitude, with the gate bias voltage being fixed. Namely, the high frequency power amplifier circuit produces a too small output current variation relative to the DC bias component, particularly at a small output level, and a resulting poor sensitivity of output level detection disables accurate output control and invites the fluctuation of detection level in response to the temperature variation and power voltage variation.
It is an object of the present invention to provide, for a wireless communication system having both of phase modulation and amplitude modulation, a high frequency power amplifier circuit and an electronic component part (module) incorporating the circuit which are capable of reducing the circuit scale by eliminating the need of an amplifier circuit dedicated to vary the input signal amplitude to meet the demanded output level.
Another object of the present invention is to provide, for a wireless communication system having both of phase modulation and amplitude modulation, a high frequency power amplifier circuit and an electronic component part incorporating the circuit which are capable of alleviating the signal leakage to the reception frequency band by lowering the gain of amplifier circuit during a linear operation.
Still another object of the present invention is to provide, for a wireless communication system having both of phase modulation and amplitude modulation, a high frequency power amplifier circuit and an electronic component part incorporating the circuit which are capable of detecting the output level to be fed back based on the current sensing scheme and capable of having a sufficient detection sensitivity even at a small output level thereby to perform accurate output control.
These and other objects and novel features of the present invention will become apparent from the following description and attached drawings.